OUTCROP FRACTURE CHARACTERIZATION OF PENNSYLVANIAN JACKFORK GROUP SANDSTONES IN THE CENTRAL OUACHITA MOUNTAINS, SOUTHEASTERN OKLAHOMA

The Ouachita Mountains in southeastern Oklahoma and southern Arkansas represents the exposed portion of a >2000 km long orogenic system that includes the Appalachians as well as the Marathon thrust belt in Texas. This study focuses on the Central Ouachitas in Oklahoma, which is composed of primarily Mississippian to Pennsylvanian clastic rocks that are deformed into broad, open synclines separated by tight anticlines and a few thrust faults. The major exposed interval in the Central Ouachitas is the Pennsylvanian Jackfork Group, a deepwater turbidite deposit. Economically, the Jackfork Group plays an important role in the subsurface in Oklahoma as a tight gas reservoir interval, including the very prolific Potato Hills field. Here we report initial results on characterizing outcrop scale fractures in the Jackfork. This work is part of a larger structural and stratigraphic study focused on fracture processes in deepwater turbidites. As such, this research has implications for diverse areas ranging from the tectonic development of the Ouachita system to exploration in tight, natural gas reservoirs.

Fracture orientation, spacing measurements, and trace lengths were collected via standard scanline techniques on several Jackfork bedding surfaces exposed in the north limb of the Lynn Mountain syncline south of Big Cedar, Oklahoma. Both stratigraphic and mechanical bed thickness were recorded for each surface. Two primary fracture orientations were observed. The more dominant set, in terms of greater trace length, is nearly strike-parallel and bed-perpendicular with an average orientation of 104/60N. The secondary set is dip-parallel and bed-perpendicular with an average orientation of 017/89W. Our field observations show that the second set has a shorter average trace length and generally terminates against the first, although exceptions were observed. Both fracture sets show positive correlations between fracture spacing and bed thickness (particularly mechanical bed thickness) such that thicker beds exhibit larger spacings. Further work will focus on refining the areal fracture density, outcrop-scale connectivity, and also the relationship between lithology and fracture characteristics. As such, this research is an integral part of ongoing research into the tectonic development of the Ouachita orogenic system.